Chapter 9: Central Nervous System Flashcards
two divisions of the nervous system
1) central nervous system
- brain and spinal cord
- is the integrating centre
2) peripheral nervous system
- all neural tissue outside of the CNS
what are neurons?
neurons are cells that send and receive electrical signals rapidly
role of central nervous system
- consists of the spinal cord and the brain
- integrates information from internal/external environment to illicit appropriate responses for the maintenance of homeostasis
role of peripheral nervous system
- includes all neural tissue outside the CNS
- provides communication between the CNS and the rest of the body
- has two divisions: somatic and autonomic
What is CSF and why is it important?
- cerebrospinal fluid is a clear watery fluid that bathes the brain and spinal cord (the CNS)
- it is important because it helps with protection, nourishment, and waste removal in the brain
main functions of CSF
- CSF cushions the brain (acts as a shock absorber) by preventing the soft nervous tissue from colliding with the hard skull
- CSF helps maintains a stable interstitial fluid environment because it provides neurons and glial cells with essential nutrients and helps with removal of waste products
where does CSF come from?
- it is secreted by the ependymal cells of the ventricles in brain
- CSF is then circulated throughout the four ventricles
how much blood supply is sent to the brain? why so much?
- the CNS receives 15% of the blood that the heart pumps
- a large blood supply is necessary because the CNS tissue has a high rate of metabolic activity compared to most other body tissues
- therefore, it has a high demand for glucose and oxygen to meet its energy needs
what happens if there inadequate blood flow to the brain?
- can result in irreversible damage to CNS tissue
- can cause deficits in certain functions like the ability to speak or move an arm
why is blood flow to the brain critical?
- to help meet the brains energy requirements
- blood flow provides oxygen and glucose to the brain which is needed needs to function properly and stay alive
- neurons in CNS depend on aerobic glycolysis (which requires oxygen and glucose) to maintain ATP levels therefore requiring a steady supply from the blood
what are the different energy supplies to the brain?
- GLUCOSE and oxygen are the main energy supply
- ketones can be used for energy during extreme conditions when glucose supply is limited
why is the CNS so sensitive to changes in blood supply?
it is because the brain and spinal cord have high metabolic demands and limited energy reserves.
- the brain has limited glycogen stores so cells in the CNS must obtain glucose directly from the blood instead
- cells in the CNS do not have access to fatty acids reserves for energy, which increases their demand for glucose
- neurons are more sensitive to oxygen deprivation than other kinds of cells with lower rates of metabolism (because they cannot perform anaeorbic glycolysis)
what are capillaries?
- they are the site of exchange between blood and interstitial fluid
- they’re composed of a thin, single layer of endothelial cells (a type of epithelial cell)
- small wall = short diffusion distance for exchange
what is blood-brain barrier?
- a physical barrier that is selectively permeable
- the capillaries in the brain create protective barrier that restricts the exchange of solutes between blood and brain
how does the blood brain barrier work?
- barrier is the result of tight junctions between capillary endothelial cells
- they eliminate capillary pores which restricts the diffusion of molecules between the cells
why are astrocytes important to the formation of the blood-brain barrier?
- they stimulate endothelial by releasing chemical signals that promote the formation of tight junctions between the endothelial cells of the brain’s blood vessels
importance of the blood-brain barrier
- protects the CNS from harmful substances that may be present in the blood by restricting movement of molecules from the blood stream to the brain
- in order to move across the barrier (enter or leave capillaries) the molecules must cross the endothelial cells themselves.
what is the spinal cord?
- a cylinder of nervous tissue
- is continuous with the lower end of the brain and is surrounded by the vertebral column
- is the origin of spinal nerves (31 pairs)
what is organization of white and gray matter in the spinal cord?
gray matter = concentrated within a butterfly-shaped region in the middle of the cord
white matter = found in the surrounding outer region
what is organization and role of gray matter in the spinal cord?
- divided into dorsal (posterior) and ventral (anterior) regions
- dorsal region = sensory functions / afferent
–> afferent neurons originate in the periphery as sensory receptors and terminate in the dorsal horn - ventral region = motor functions / efferent
–> efferent neurons originate in the ventral horn and travel to the periphery, where they form synapses with skeletal muscles
what is organization and role of white matter in the spinal cord?
- white matter of the spinal cord form tracts that provide communication between the brain and spinal cord
–> ascending tracts = transmit information from spinal cord to brain
–> descending tracts = transmit information from brain to spinal cord
what is a reflex?
an automatic, patterned response to a sensory stimulus
what is a reflex arc?
- a neural pathways for reflexes
3 reflex pathways
1) stretch reflex (i.e knee jerk)
2) withdrawal reflex
3) crossed-extensor Reflexes
5 components of a reflex arc
1) sensory receptor
2) an afferent neuron
3) integrating centre
4) an efferent neuron
5) an effector organ
how does the reflex arc work?
- the receptor first detects a stimulus
- information is then transmitted from the receptor to the CNS via the afferent neuron
- the CNS, which functions as the integrator, sends signals via the efferent neuron
- the efferent neuron transmits signals to the effector organ which stimulates it to produce a specific response.
–>the integration can consist of a single neuron or a network of neurons
4 classifications of reflexes
1) spinal or cranial
–> based on the level of neural processing involved
2) somatic or autonomic
–> depends on which efferent division controls the pathway
3) innate or conditioned
–> whether you’re born with the reflex or learned through experiences
4) monosynaptic or polysynaptic
–> monosynaptic reflexes = the neural pathway consists of only twoneurons and a single synapse
–> polysynaptic reflexes = contain more than two neurons and multiple synapses
what is the muscle spindle stretch reflex?
a rapid and automatic response to changes in muscle length
how does the stretch reflex work using the knee-jerk test?
- tapping the patellar tendon below the kneecap stretches the quadricep muscle. this causes a change in muscle fibre length.
- muscle spindles (which are sensory receptors) the muscle detect a change and get excited
- this triggers action potentials to travel in afferent neurons to the spinal cord (acting as an integration center)
- in the spinal cord, the afferent neurons make direct excitatory synaptic connections with efferent neurons that innervate the quadriceps muscle
- this stimulates the quadriceps to contract and the leg to “kick” forward
- the afferent neurons also synapse with inhibitory interneurons that innervate hamstring motor neurons allowing it to relax
what is a withdrawal reflex?
when a portion of the body is subjected to a painful stimulus, it withdraws from the stimulus automatically
what is the crossed-extensor reflex?
a contralateral reflex, meaning the reflex occurs on the opposite side of the body from the stimulus
how does the withdrawal reflex work?
- begins when specialized sensory receptors, such as pain receptors (nociceptors) in the skin, detect a harmful or painful stimulus
- the sensory/ afferent neurons transmit signals from the site of the stimulus to the spinal cord (integrating centre).
- the sensory/ afferent neurons enter the spinal cord and have an excitatory synapse with interneurons.
- the interneurons in the spinal cord excite and relay signals to the motor/efferent neurons that innervate the skeletal muscles that cause withdrawal of the limb
- these motor neurons stimulate the contraction in the affected limb.
- the contraction pulls the limb away from the harmful stimulus, allowing for a rapid and protective withdrawal.
- to facilitate the withdrawal, there is often a simultaneous relaxation of antagonistic muscles (those opposing the action), allowing for a quick and coordinated movement.
how does the crossed-extensor reflex work?
- begins with the detection of a painful stimulus in one limb which activates pain receptors (nociceptors) in the affected area
- sensory/ afferent neurons carry the pain signals from the stimulated limb to the spinal cord (integrating centre) where they synapse with interneurons.
- the interneurons have branches that can relay signals to motor/efferent neurons on both sides of the spinal cord which creates a coordinated response involving both the stimulated and opposite limbs.
- on the side where the painful stimulus occurs, motor/efferent neurons signal the muscles to contract (withdrawal reflex). This causes the withdrawal of the limb from the stimulus.
- simultaneously, on the opposite side of the body, motor/efferent neurons stimulate muscles to extend which helps support the body’s weight and maintain balance during the withdrawal of the other limb.
